There already exist on the marketplace and in use a number of electrosurgical instruments which employ electron tubes to generate the desired radio-frequency (RF) currents. Ellman U.S. Pat. No. 3,730,188, whose contents are herein incorporated by reference, discloses one form of instrument that has been in wide use for many years. In the circuitry described in this patent, when the power ON-OFF switch on the instrument front panel is thrown ON, power from the conventional alternating current (AC) source or line activates the direct current (DC) power supply for the RF generator and turns ON the filament circuit for the electron tube used in the RF oscillator section to generate the RF currents. The choice of currents available for cutting, coagulation or fulguration is determined by a rotary switch on the instrument front panel which selectively introduces rectifiers and capacitors into the DC power supply circuit to control the waveform of the DC voltage applied to the plate and screen grid of the tube. However, in this standby mode, no RF currents are actually generated because the tube's cathode circuit to ground remains open due to the series connection of a normally-open foot switch which is plugged into a jack on the instrument's front panel. To place the unit in its operating mode, the user (dentist or doctor) presses the foot switch. This closes the tube's cathode circuit and, since the cathode filament is hot, immediately causes conduction through the tube activating the RF oscillator. The RF currents thus generated are conducted via a cable plugged into a jack on the instrument front panel to the handpiece held by the user and to the probe tip, which now may be applied by the user to the patient to perform an electrosurgical procedure. Releasing the foot switch returns the unit to its standby mode.
Recently, there has come on the market modified handpieces with built-in or incorporated finger switches which function to switch power delivery to the handpiece tip thus eliminating the foot switch. One such unit is described in U.S. Pat. No. 4,034,761. A commercial version is fitted at the cable end with a 3-pin plug connector adapted to plug into three similarly configured jacks on the instrument front panel. One pin carries the RF current, and the remaining two pins perform the instrument ON-OFF switch function controlled by one or two finger switches on the handpiece.
U.S. Pat. No. 4,071,028 describes a typical solid-state instrument usable with this new finger-switch handpiece. It comprises three separable units: The mainframe containing a solid state RF generator and power supply therefor, a control unit that plugs into the mainframe, and the finger-switch handpiece with cable that plugs into the control unit. The foot switch is eliminated. The control unit contains a light-emitting diode (LED) and battery in series with the finger switch. When the user actuates the finger switch, the battery turns on the LED. The LED light, detected by a photoconductor to form a photon coupled isolator, in turn turns on a power TRIAC whose anode-cathode circuit is connected in series with the AC line leading to the mainframe. This, in turn, conducts AC power from the line into the mainframe, turning on the solid-state RF generator, whose current is conveyed via the control unit to the handpiece.
The above-described combination is not suitable for use with a tube-type RF generator, because if connected to a tube mainframe, a delay in the production of the RF currents would ensue as a result of the delay in heating of the tube filaments. It typically takes about 30 seconds warm-up time before the tube filaments reach operating temperature and sufficient tube conduction results to sustain the RF oscillations. A user cannot efficiently use such a combination if he must wait 30 seconds after operating the finger switch before the handpiece can be applied to the patient.